CULT: Fungal and Bacterial Leaf Spot

In a message dated 9/10/05 7:25:50 AM Eastern Daylight Time, 
lmann@volfirst.net writes:

<<  I suspect bacterial spot (i.e., Erwinia) is more of a problem farther 
south, fungal farther north. >>

According to 'The World of Irises', Erwinia carotovora is the pathogen for 
Bacterial Soft Rot, of which one symptom may or may not be leaf discoloration. 

The pathogen for Bacterial Leaf Spot is Xanthomonas tardicrescens seen as: 
"Large irregular spots mostly near the margin of the upper leaf appear first as 
water-soaked areas that quickly turn brown. The spot lengthens downward 
following veins in the leaf and adjacent spots may run together. In severe 
infestations the entire upper leaf may die. Easily confused with fungal leaf spot 
disease. The spots caused by this bacterium are larger, more irregular, and more 
confined to the leaf-margins than those of the fungal leaf spot.

There is a black and white illustration, and I'd call the bacterial spot 
bacterial streak, myself. The following-the-veins bit is very apparent.

The pathogen for Fungal Leaf Spot is Didymellina macrospora, known also, and 
more widely in my experience, as Heterosporium gracilis, although "several 
other fungi...have been reported as causative agents of leaf spot or leaf 
infections in irises...." and there follows some discussion of these which I am not 
going to dwell upon at this point. 

The typical manifestation of fungal leaf spot is "Small circular to oval 
spots looking watery or greasy appear on the leaves. After a day or two these 
enlarge to 3-6 mm ( 0.125-0.25 inch) diameter, turn yellowish to brown, and often 
develop a distinctive red brown border. If plants are severely infected, 
leaves may die back from the tip...." Follows discussion that seasonal growth may 
be affected but the "disease is not lethal." Here the picture shows distinct 
little spots with rings. 

Bill Shear, in his 'The Gardener's Iris Book', opined that fungal spot was 
more prevalent than bacterial spot, and he offers a nice color photo of the 
former, with the distinct redbrown rings showing. It seems to me if you first 
treat diligently for fungal spot, and see no results within a season, and dramatic 
improvement in two years, then you might consider the possibility of 
bacterial spot. 

'The World of Irises', published in 1978, is still available from the AIS 
Storefront at a clearance price. There is a good deal of talk about the book 
being stale and I think some of that concern is legitimate, and some of it is not. 
Nor was the book everyone's cup of tea to begin with. I, myself, do not care 
for the way it reduces the entire history of garden irises to the history of 
hybridizing, and I have often heard it said that the systematic botany at the 
beginning is mighty hard slogging if you don't know much to begin with. It is 
very much an American book. Notwithstanding its age and clear limitations, this 
is a pretty good book, a very important book, a book which did not dumb the 
hard stuff down more than it had too. It is a book thoughtful people interested 
in irises will still wish to have on their reference shelf.

The AIS Storefront now appears to be located under Member Services on the AIS 
webpage at <<www.irises.org>>. I had to go looking around for it just now and 
I only found it because I knew it had to be there. I strongly suggest that 
its link needs to be up front and clearly visible on the AIS home page so 
visitors will see it immediately. I do not think the Storefront is an AIS Member 
Service; we want to sell stuff to everyone. 

Cordially,

Anner Whitehead
Richmond VA USA 

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